INDEFENSIBLE: Kirk Nieuwenhuis (top) goes over shortstop Omar Quintanilla as they try to field a hit by Nationals pitcher Jordan Zimmermann during the third inning of the Mets’ 7-6 loss in 12 innings last night. Photo: AP

WASHINGTON —These are the kind of losses that must cause manager Terry Collins to invoke the name of Triple Crown hopeful “I’ll Have Another.”

Last night, it was a combination of bullpen and defense that sunk the Mets. Forced to use Jordany Valdespin at shortstop for a stretch, Collins watched the rookie commit two huge 10th-inning errors that tied the game before Washington’s Bryce Harper delivered an RBI single against Elvin Ramirez in the 12th, sending the Mets to a 7-6 loss at Nationals Park.

“The toughest part is the way we fought back,” Collins said. “We were down three, we fight back. We get the lead, we lose the lead and to lose the game was really tough.”

Scott Hairston homered in the 12th to give the Mets a 6-5 lead before Michael Morse and Ian Desmond stroked consecutive doubles in the bottom of the inning to tie the game. Then, with the bases loaded and two outs, the phenom Harper singled to left against Ramirez, ending the 4-hour, 15-minute marathon. The loss denied the Mets a chance to move alone into first place in the NL East.

“It was one of those games, it was going back and forth and we had a couple of unfortunate situations on the field where [plays] didn’t go our way,” Hairston said. “I guess we’ll have to learn from it.”

Ahead 5-4 after Hairston had scored on a wild pitch in the 10th inning, the Mets couldn’t close the deal in the bottom of the frame as Valdespin — inserted at shortstop — booted two grounders that helped the Nationals tie the game. Ryan Zimmerman reached first on a Valdespin grounder to begin the inning and scored the tying run against Bobby Parnell when Valdespin booted Ian Desmond’s grounder.

“I feel bad because we lost the game and we made errors, but I’m trying to make the play,” Valdespin said.

The Mets took a 4-3 lead into the bottom of the eighth, but watched the Nationals tie the game on Desmond’s RBI single against Frank Francisco. Zimmerman singled leading off the inning against Tim Byrdak and Adam LaRoche followed with a grounder that Daniel Murphy momentarily booted, costing him a chance to go for the lead runner at second. Murphy threw out LaRoche, but the bobble proved big when Desmond delivered his two-out single.

Andres Torres’ two-run double in the eighth against Craig Stammen gave the Mets their first lead at 4-3. Valdespin’s double with one out and Hairston’s walk started the rally.

In his return to the majors after undergoing surgery to repair a torn anterior capsule in his shoulder 13 months ago, starter Chris Young lasted five innings and allowed three runs, two earned, on six hits with two strikeouts and one walk. He was removed after 75 pitches.

“I was just hoping I wouldn’t get overly excited where I throw the ball all over the place,” Young said. “I wanted to stay calm and hit my spots and work ahead in the count and for the most part I did that.”

As planned, Collins then used Miguel Batista for two innings. The veteran righty, in his return from the disabled list, gave the Mets two scoreless innings with two strikeouts.

David Wright’s seventh homer of the season, a solo shot in the sixth, pulled the Mets within 3-2 after Valdespin had homered earlier in the inning. For Wright, the blast gave him 736 runs scored in a Mets uniform, putting him one ahead of Jose Reyes for No. 1 in franchise history.

Valdespin’s homer leading off the sixth against Jordan Zimmermann got the Mets on the board after the Nationals had scored twice in the previous inning. Valdespin, pinch hitting for Young, hammered a shot into the right-field bullpen, for his second homer of the season.

The Nationals scored twice in the fifth to take a 3-0 lead. Zimmerman’s RBI single accounted for the first run, and after a brief rundown shortstop Omar Quintanilla fired a throw over catcher Josh Thole’s head, allowing Steve Lombardozzi to score.